This invention relates to an fixative for glass fibers and to a glass fiber fixing process. The fixing process and fixative enable glass fibers such as are commonly used in optical communications components to be fixed in position with a hermetic seal.
Optical fibers are relatively thin, fragile strands of glass along which an optical signal propagates. For example, see FIG. 1 of the accompanying drawings for a cross-section of a typical optical fiber. An optical fiber 2 typically comprises an glass fiber core 4 covered by a glass cladding material 6. Typically a primary coating 8a, and secondary outer coating 8b are then used to protect the fiber. The outer coating protects the glass fiber and increases the robustness of the optical fiber.
These outer coatings are typically formed from polymeric materials which have low melting points relative to the glass cladding and core, for example acrylic materials and materials such as thermoplastic polyester, for example Hytrel™. Before the optical fiber can be bonded to another surface, the outer coating must be removed to expose the optical fiber core in the region in which a bond is to be formed. This enables a stronger bond to be formed between the glass fiber material and the other surface element to be bonded.
Forming a strong bond between an optical fiber and an optical component is important as many optical components are subject to vibration. Several problems are associated with the bonding process between a glass material and a non-glass material generally, and the high design specifications for optical components exacerbate these problems.
For example, temperature variations require any bond formed ideally to match the thermal coefficients of expansion of the optical fiber and the bonded part to mitigate thermal stress on the optical fiber. Determining the composition of a glass fixative having a sufficiently low melting point to enable an optical fiber to be bonded to a non-glass material without deforming the optical fiber, having a desired thermal coefficient of expansion, and good adhesive properties to both the silica of the optical fiber and the non-glass material is a difficult and complex task. Alternative processes using solder compounds such as Sn/Pb alloys were employed instead.
Most optical components include parts which have a non-glass composition. For example, metallic materials such as Kovar. One known method of bonding glass to such metallic materials requires the glass fiber to be metalised. The metalisation process required a fiber to be stripped to its core and given a metallic coating consisting of a bonding layer and a soldering layer. This enables bonds between the metalised glass fiber and the Kovar material to be soldered.
Metallisation processes have several disadvantages. The fibers have to have their adhesion verified and any masking material used must be removed. Such metallisation processes are time consuming and the fiber strength can be significantly reduced as a result (typically for example by 30%). Other disadvantages include the extensive handling of fibers required by such processes and the associated high fiber breakage rate, and the capital expenditure on plant required by such processes. A further disadvantage of metalisation processes for fiber fixing is that the soldering process can leave behind potentially corrosive fluxes. The preparation of the fibers for metallisation and soldering is moreover time-consuming. Yet another disadvantage of fiber fixing using metalisation processes is that no reworking is possible during either the metalisation or soldering stages.
In complex optical components, a further problem is the necessity of preventing preexisting bonds from being degraded when subsequent bonds are formed in the near vicinity.
The bonds must be sufficiently strong and intact form a hermetic seal between the optical fiber and the corresponding portion of the optical component to isolate the interior of the optical component from the external atmosphere. This enables the atmosphere within the optical component to be isolated and for non-air atmospheres or pure air atmospheres to be used. Moreover, the moisture content of the interior can then be controlled. It is thus important for any bond formed to be sufficiently strong to retain the hermiticity of the seal when subject to thermal stress and/or vibration and/or shock.